Tantalum sputtering target and method of manufacture

ABSTRACT

Described is a method for producing high purity tantalum, the high purity tantalum so produced and sputtering targets of high purity tantalum. The method involves purifying starting materials followed by subsequent refining into high purity tantalum.

FIELD OF THE INVENTION

This invention relates to a method and apparatus for producing highpurity tantalum and the high purity tantalum so produced. In particular,the invention relates to production of high purity tantalum.

BACKGROUND OF THE INVENTION

Tantalum is currently used extensively in the electronics industry whichemploys tantalum in the manufacture of highly effective electroniccapacitors. This is mainly attributed to the strong and stabledielectric properties of the oxide film on the anodized metal. Bothwrought thin foils and powders are used to manufacture bulk capacitors.In addition, thin film capacitors for microcircuit applications areformed by anodization of tantalum films, which are normally produced bysputtering. Tantalum is also sputtered in an Ar—N₂ ambient to form anultra thin TaN layer which is used as a diffusion barrier between a Culayer and a silicon substrate in new generation chips to ensure that thecross section of the interconnects can make use of the high conductivityproperties of Cu. It is reported that the microstructure andstoichiometry of the TaN film are, unlike TiN, relatively insensitive tothe deposition conditions. Therefore, TaN is considered a much betterdiffusion barrier than TiN for chip manufacture using copper asmetallization material. For these thin film applications in themicroelectronics industry, high purity tantalum sputtering targets areneeded.

Most of the tantalum metal produced in the world today is derived fromsodium reduction of potassium heptafluotantalate (K₂TaF₇). Processeswhich are not adapted commercially to any significant extent include thereduction of tantalum oxide (Ta₂O₅) with metallic reductants such ascalcium and aluminum, and non metallic reductants carbon and carbonnitrogen; the reduction of the tantalum pentachloride (TaCl₅) withmagnesium, sodium or hydrogen; and the thermal dissociation of TaCl₅.

Reduced tantalum is obtained either as powder, sponge or massive metal.It invariably contains significant amounts of oxygen, as well as otherimpurities such as reductants and impurities that may be present in thestarting tantalum compounds. For removal of impurities in tantalum,electron beam melting is often conducted. During electron beam melting,most of the metallic impurities and interstitial gases are vaporizedbecause of their high vapor pressure at the melting point of tantalum(2996° C.). Essentially all elements, except niobium, tungsten,molybdenum, uranium and thorium can be eliminated this way. While themetallic impurities and nitrogen are removed by direct volatilization,the removal of oxygen takes place via mechanisms involving formation andevaporation of carbon oxides, aluminum oxides, water, as well assuboxides of tantalum. The purity can be further improved by repeatedelectron beam melting. Other refining processes include vacuum arcmelting, vacuum sintering, molten salt electrorefining and tantalumiodide refining, with the iodide process being the most promisingtechnique for removing tungsten and molybdenum.

The above mentioned refining methods are not effective for removal ofniobium from tantalum. Since tantalum and niobium are closely associatedwith each other in nature, the removal of niobium is critical to preparevery high pure tantalum. In practice, their separation is conductedbefore reduction via methods such as solvent extraction, chlorinationand fractional crystallization.

The tantalum target manufacturing process includes forging ingot intobillet, surface machining billet, cutting billet into pieces, coldrolling the pieces into blanks, annealing blanks, final finishing andbonding to backing plates.

SUMMARY OF THE INVENTION

In accordance with the present invention there is provided a method andapparatus for producing high purity tantalum sputtering targets and thehigh purity tantalum so produced.

The method comprises purifying potassium heptafloutantalate, K₂TaF₇,reducing the purified K₂TaF₇ to produce tantalum powder, refining thetantalum by reacting with iodine and finally electron beam melting thetantalum to form a high purity tantalum ingot.

The starting material is commercial K₂TaF₇ salt, made by dissolvingtantalum ores in hydrofluoric and sulfuric acid mixture, followed byfiltration, solvent extraction using methkylisobutylketone (MIBK) andcrystallization of K₂TaF₇. This can be repeated several times to lowerthe impurity levels, in particular the level of Nb.

Sodium reduction of purified K₂TaF₇ is conducted in a liquid liquidreduction retort where K₂TaF₇ and diluents (KCl and NaCl) are heated toabout 1000° C. Molten sodium is then injected into the retort forreacting with K₂TaF₇. Agitation of the reactants is provided toaccelerate the reduction reaction. After cooling, the mass is taken outof the retort, crushed, leached and washed to separate tantalum powderfrom the salt mixture.

Tantalum refining is done by the iodide process or electron beammelting. These methods can be used in parallel or in series. Electronbeam melting is preferred as the last step because it results in aningot which is suitable for further physical metallurgical steps towardthe goal of target manufacture.

Electron beam melted ingot is forged into billets and surface machined.After surface machining, the forged billet is cut into pieces, which arefurther cold rolled into blanks. Blank annealing is carried out in aninert atmosphere to obtain a recrystallized microstructure. The blanksare then machined to obtain a final finish and bonded to copper oraluminum backing plates.

For characterization of targets produced by the invented process,chemical analyses are conducted. The methods of chemical analysis usedto derive the chemical descriptions set forth herein are the methodsknown as glow discharge mass spectroscopy (GDMS) for metallic elementsand LECO gas analyzer for non metallic elements.

The highly purified tantalum material of the present invention has lessthan 500 ppm by weight, total metallic impurities, an oxygen content ofless than about 100 ppm, by weight, a molybdenum or tungsten content ofnot more than 50 ppm, by weight, and a uranium and thorium content ofnot more than 10 ppb, by weight. It is also possible to produce tantalumhaving less than 5 ppm, by weight, total of molybdenum and tungsten.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of the liquid liquid reaction retort usedfor sodium reduction of K₂TaF₇.

FIG. 2 is a schematic diagram of an iodide cell;

FIG. 3 is a schematic diagram illustration an iodide cell with adistillation unit;

FIGS. 4A and 4B are schematic diagrams of a tantalum target; and

FIG. 5 is a graph of conductance of tantalum bar as a function of time.

DETAILED DESCRIPTION

1) Precursor Purification and Sodium Reduction

In nature, tantalum generally occurs in close association with niobium,tin and other elements. The minerals most commonly used as raw materialsin tantalum production are Tantalite, Wodginite, Micolite andSamarskite. These minerals are enriched by wet gravity, magnetic orelectrostatic methods. The concentrates are dissolved in a mixture ofhydrofluoric and sulfuric acid. The resulting solution is filtered, thenseparated from niobium and other impurities in a solvent extractionplant. The tantalum concentrate is transferred into an aqueous solutionand precipitated with ammonia to yield tantalum acid (Ta₂O₅xH₂O),calcined at an elevated temperature to yield tantalum oxide.Alternatively, the tantalum is crystallized to potassiumheptafloutantalate, by addition of KF and KCl to the hot aqueoussolution obtained from solvent extraction. Impure potassiumheptafloutantalate obtained by these methods must be further purifiedfor use as a source of tantalum for the electronics industry.

In general, potassium heptafloutantalate may be purified by a proceduresuch as follows:

Technical grade potassium heptafloutantalate (K₂TaF₇) is dissolved inHF, e.g. a 49% HF solution. A mixture of HF and H₂SO₄ can also can alsobe used for the dissolution process. The amount of K₂TaF₇ dissolveddepends on the temperature and concentration of HF. Since thedissolution rate is very slow at room temperature, the mixture is heatede.g. to 90° C. in a suitable container. The solution containing K₂TaF₇is covered, to prevent losses due to evaporation, and stirredcontinuously. Time to dissolution is approximately one hour. A 65° C.KCl solution is added to the K₂TaF₇ solution and the resulting solutionis stirred while cooling to room temperature. The tantalum in solutionprecipitates as K₂TaF₇ since the solubility of K₂TaF₇ is very low atroom temperature. The precipitate is filtered, washed and dried.Niobium, tungsten, molybdenum, zirconium, uranium and thorium remain insolution. Repeated dissolution and precipitation may be useful in orderto obtain extremely high purity tantalum. Elements such as niobium,tungsten, molybdenum, uranium and thorium, which are difficult to removeby electron beam melting, are easily removed by this process.

Potassium heptafloutantalate can be reduced to tantalum metal by fusedsalt electrolysis or reduction by sodium. The rate of reduction byelectrolysis is very slow, therefore sodium reduction is used forprocessing large quantities of K₂TaF₇ into tantalum metal. The overallreduction reaction can be written asK₂TaF₇+5Na(1)=Ta(s)÷2KF+5NaF   (1)

Referring to the drawings, FIG. 1 shows a reduction furnace. Thereduction is carried out by placing K₂TaF₇ and some dilute salts such asKCl, NaCl, LiCl, CsCl, CaCl₂, etc. into a reactor equipped with astirring device. The reactor is placed in a furnace heated to above themelting point of the salt mixture, usually under 1000° C. Molten sodiumis injected into the reactor and stirred while controlling thetemperature. After cooling, the mass is removed from the reactor,crushed and leached with a dilute acid to recover tantalum metal powder.The powder is compacted and melted in an electron beam furnace.

2) Iodide Process

Tantalum metal is produced from the reduction of commercially availableK₂TaF₇ by sodium, which is a process similar to the Hunter process usedfor the production of sponge titanium. The metal produced by thereduction of sodium contains most of the impurities that exist in theK₂TaF₇ such as Fe, Ni, Ti, W, Mo, etc. The metal is in the form ofpowder and has a very high oxygen content.

The method described herein is capable of producing high purity tantalumfrom scrap or impure tantalum metal. The process is based on chemicaltransport reactions, in which tantalum iodides are formed by thereaction of impure tantalum metal with iodine gas (synthesis zone), atlower temperatures, then the tantalum iodides are decomposed on a hotwire filament, at higher temperatures, to produce a very pure metal(deposition or thermal decomposition zone.). The impure tantalum isconverted into gaseous species according to the following reactions inthe synthesis zone:Ta(s, impure)+5/2I₂(g)=TaI₅(g) (Synthesis reaction)   (2)Ta(s, impure)+5I(g)=TaI₅(g) (Synthesis reaction)   (3)

Similar reactions can be written for the other tantalum iodide species,such as TaI₃ and TaI₂. The gaseous species of tantalum diffuse into thethermal decomposition zone and decompose to form pure tantalum metalaccording to the following reaction:TaI₅(g)=Ta(s)+5I(a) (Thermal decomposition reaction)   (4)

The thermodynamic factors are important to understanding and controllingthe process. Thermodynamic calculations have been carried out todetermine advantageous operating conditions, such as temperature andpressure, in the synthesis and decomposition zones.

A schematic diagram of the apparatus is shown in FIG. 2. The processapparatus contains a cell, filament and feed material and is designed torun batch operations. After each run the apparatus is cooled to roomtemperature and disassembled.

The preferred iodide cell, for the refining of tantalum, is an alloy 600(Inconel) container clad with a metal more electrochemically noble thantantalum according to the chloride electromotive series, such asmolybdenum or tungsten or an alloy thereof The cladding preventscontamination of the refined tantalum by cell components sincemolybdenum and tungsten do not react with iodine at cell operatingtemperatures. Alloy 600 (Inconel) containers are also used for therefining of metals such as Ti and Zr, without cladding, since thesemetals are refined under different operating conditions.

A filament made of pure tantalum rod is used for the decompositionsurface. The filament can be in the shape of a U or can be a differentshape to increase its surface area. It is also possible to use multiplefilaments to increase the surface area and cell productivity. Thefilament is heated resistively by an external power supply. Since thefilament temperature affects the deposition rate, the current iscontrolled to maintain the filament temperature between 1000 and 1500°C. Tantalum crystals then grow on the filament.

A cylindrical molybdenum screen is placed in the cell to provide anannular space 1 to 3 inches wide. The annular space if filled withtantalum feed material in the form of chips, chunks or small pellets.This type of arrangement gives a high surface area for the reactionbetween feed material and iodine gas in the cell. The crude tantalum canalso be compacted to a donut shape and placed in the reactor. The feedmaterials are cleaned with cleaning agents before they are charged intothe cell.

A good vacuum system is advantageous to producing tantalum with lowimpurities. Therefore, the cell is connected to a vacuum systemproducing 1 micron or less of pressure. The cell is evacuated at roomtemperature, then heated to around 800-1000° C. under vacuum to removeall the volatile impurities before iodine is added.

The temperature in the synthesis zone effects the rate of reaction. Thetemperature in the synthesis zone should be uniform and kept much higherthan boiling point of TaI₅. A special heater placed on the lid of thecell keeps the temperature at around 350-500° C., which prevents thecondensation of iodides under the lid. Without this heater, iodine mustbe continuously added to the system.

Oxygen in tantalum originates from numerous sources, starting with theprecursor and on through electron beam melting. Oxygen is undesirable athigh concentrations due to its effect on the resistivity of depositedtantalum thin films. Currently available methods cannot easily decreasethe oxygen levels to less than 30 ppm. Thermodynamic calculations, aswell as the experimental results, indicate that the metal oxides formedor present in the feed material do not react with iodine and are nottransported to the decomposition zone. Therefore, this process iscapable of producing high purity tantalum with very low oxygen. Theamount of oxygen remaining in the cell atmosphere is reduced by acombination of argon flushing and vacuum. Nitrogen in the feed materialbehaves like oxygen, therefore the nitrogen content of tantalum crystalbar will be very low.

Electron beam melting is frequently used to refine tantalum. However,electron beam melting cannot remove elements such as tungsten andmolybdenum, since the vapor pressures of these elements are very low atthe melting temperature of tantalum. The present process is capable ofconsistently removing elements such as tungsten and molybdenum toextremely low levels. The process may also remove uranium and thorium,which cannot be removed by electron beam melting.

The iodide process described above may not be able to remove significantamounts of niobium. Therefore, the current process has been modified toobtain pure tantalum with very low metallic impurities includingniobium. In the modified process, tantalum scrap or crude tantalum isreacted with iodine gas to form gaseous TaI₅ and NbI₅, which then areseparated by fractional distillation, since the boiling points of thesetwo compounds are different. A schematic of the apparatus is shown inFIG. 3.

Crude tantalum or scrap is placed in a vertical tube made of Inconel andclad with molybdenum, tungsten or an alloy thereof. The tube is placedin a furnace that is heated to 400-700° C. A carrier gas such as cleanargon or helium is passed over an iodine bath. The temperature of thebath is adjusted to get a specific I₂ partial pressure. Iodine gasreacts with tantalum scrap to produce gaseous tantalum and niobiumiodide. The gas from the feed reactor passes through the distillationcolumns. The temperature of the first column is maintained just belowthe boiling point of TaI₅, to condense TaI₅. The second column ismaintained at a temperature low enough to condense NbI₅, but above theboiling point of I₂. The iodine gas is circulated through the process ofreuse. All the gas lines between the first column and furnace are madeof molybdenum and maintained at about 600° C., the others are maintainedat lower temperatures.

The pure liquid or solid TaI₅ obtained from the fractional distillationunit is fed into the deposition rector and the TaI₅ decomposes on a hotsurface to produce pure tantalum crystals. Tantalum obtained from thisprocess is very pure and free of all impurities that cannot be removedby conventional processes. The pure tantalum obtained by the modifiediodide process is electron beam melted to produce high purity tantalumingots.

3. Electron Beam Melting

Electron beam melting is commonly used to melt and refine refractorymaterials.

The process is based on the use of the intense heat generated when ahigh energy particle stream impinges on a material, transforming itskinetic energy into thermal energy. The flexibility to distribute energyyields a large number of electron beam melting techniques such asbutton, drip, hearth, zone melting, etc. for various metals. Electronbeam hearth melting has been established for titanium and super alloys.Electron beam drip melting may be used for refractory materials. Anelectron beam drip melting furnace compromises a horizontal bar feederfor primary feedstock. The bar feeder is equipped with a vacuum valvewhich allows nearly continuous feeding and melting of precompactedmaterials. The refining of refractory metals occurs via vaporization ofsuboxides, evolution and removal of gases, carbon-oxygen reaction andvaporization of metallic impurities. Most of the elements can be removedfrom tantalum during melting by the one of above mechanisms. However,electron beam melting cannot remove W, Mo, Nb, U, Th, etc. due to lowvapor pressures of these elements at the melting temperature. Repeatedmelting may be necessary to get very high purity materials.

Scrap, impure tantalum, tantalum powder obtained from electrolysis orreduction of K₂TaF₇ is compacted and melted in an electron beam dripmelting furnace to produce high purity tantalum ingots.

4. Target Manufacturing

Ingots obtained from electron beam melting are forged into billets andsurface machined. After surface machining, the forged billet is cut intopieces, which are further cold-rolled into blanks. The blanks areannealed in an inert atmosphere to obtain the desired microstructure.The blanks are then machined to obtain the final finish and may bebonded to copper or aluminum backing plates. A schematic of the targetproduced is shown in FIGS. 4A and 4B.

It is desirable to perform a chemical analysis and characterization oftargets by measuring the grain size and texture. The methods of chemicalanalysis useful to derive the chemical descriptions set forth herein arethe methods known as glow discharge mass spectroscopy (GDMS) formetallic elements and LECO gas analyzer for non-metallic elements. Lineinterception method is used for grain size determination and XRD andEBSP are used to obtain texture data.

EXAMPLE 1

About 350 grams of K₂TaF₇ was added to 595 cc of HF (49%) in a Teflonbeaker. The mixture was heated to 90° C. and stirred continuously. Thebeaker was covered with a Teflon plate to prevent evaporation of thesolution. The dissolution process lasted about one hour. About 140 gramsof KCl was dissolved in 700 cc of distilled water and heated to 60 C.The KCl solution was added to the K₂TaF₇ solution and the resultingsolution was stirred for several minutes. The solution was cooled toroom temperature which caused the tantalum in the solution to beprecipitated as K₂TaF₇, since the solubility of this compound is verylow at room temperature. The precipitates were filtered and washed withKF solution (100 gr/liter H₂0) and distilled water. The powder was driedat 160° C. in a vacuum furnace, then analyzed for composition. X-raydiffraction studies were carried out on the precipitates.

Several examples were carried out according the procedure describedabove and samples were analyzed. The niobium content of K₂TaF₇ wasreduced by 50 percent after the first treatment. The results are shownin Table 1. The data shown in Table 1 indicates it is possible todecrease the niobium content of tantalum by this method. The purifiedK₂TaF₇ may be reduced by sodium. TABLE 1 K₂TaF₇ After K₂TaF₇ AfterElement Original K₂TaF₇ First Wash Second Wash Nb 4.6 <2.2 <1 Mo 0.2 0.10.1 W 4.8 1.1 <1 Zr 0.52 0.14 <0.1 Th <0.01 <0.01 <0.01 U <0.01 <0.01<0.01 Na 1100 130 50 Fe 4.8 1.2 <1 Al 2.5 1.2 S 8.7 1.1

The data in Table 1 has shown that the contents of Nb, Mo and W arelargely lowered by this method. It is well known that these elementscannot be removed from Ta metal by electron beam melting. Therefore, aremoval of these three elements from the K₂TaF₇ is beneficial toproducing very pure tantalum. Assuming all Nb, Mo and W in the K₂TaF₇will be co-reduced with Ta in the sodium reduction stage and neglectingthe existence of all other elements listed in Table 1, a simplecalculation can be made to show the influence of the K₂TaF₇ purificationon metal purity. A complete sodium reduction of 1000 g of originalK₂TaF₇ would produce 461.7 g of Ta which would contain 9.6 mg of Nb, Moand W, resulting in a metal purity of 99.9979%. When using 1000 g twicewashed K₂TaF₇, 461.7 g of Ta produced by sodium reduction, would containless than 2.1 mg of Nb, Mo and W. The metal purity would then be99.9995%.

EXAMPLE 2

An iodide cell was used to produce pure tantalum from scrap available inthe market. The cell was made of an Inconel alloy and lined withmolybdenum for the preliminary experiments. A molybdenum screen wasplaced inside the cell and Ta scrap was used to fill the gap between thescreen and the cell wall. The cell was leak checked, then evacuated tobelow 10 microns. The cell was heated to 850 C, under vacuum, toevaporate and remove organic and other volatile compounds. Then the cellwas cooled to room temperature and the precipitates on the lid werecleaned. A filament made of pure Ta was installed on the cell lid. Thecell was sealed and evacuated to below 10 microns again. The feed washeated to about 500-600° C. and the filament to 1000-1200 C. When thefeed and filament temperature were stabilized, a measured quantity ofiodine crystals were added to the reaction chamber. The current andvoltage supplied to the filament were measured continuously. From thesevalues it is possible to calculate the conductance, which is related tothe diameter of the bar. The vessel pressure, and filament and feedmaterial temperatures were controlled. Tantalum bars are successfullygrown by this method.

It was found that the temperature of the filament and feed as well asthe pressure affects the deposition rate significantly. The growth rateof Ta bar is related to conductance of Ta bar. The growth rate in termsof conductance (Mho) is shown in FIG. 5. Very high deposition rates areobtained by this method as shown in FIG. 5. Chemical analyses of theresulting tantalum bars from several runs are given in Table 2. It mustbe noted that scrap used in the experiments was not homogenous incomposition. The original composition of the tantalum feed material isalso shown in Table 2. TABLE 2 Run 1 Run 2 Run 3 Run 4 Time, hrs 79 4562 45 Weight, gr 5925 5043 7829 5969 Element Feed 3 & (ppm) Feed 1 & 2Run 1 Run 2 4 Run 3 Run 4 Nb 1200 900 505 90 185 230 Mo 6 1.2 1.7 1.31.2 W 30,000 0.28 0.19 0.2 0.25 O 100 90 308 100 60 176 N 100 <10 3 1006 4

EXAMPLE 3

Tantalum crystal bars from various runs were melted in an electron beamfurnace. The analytical results of the tantalum feed stock and themelted tantalum ingot are shown in Table 3. TABLE 3 Feed MaterialConcentration Concentration After Element (Average ppm) Melting (Averageppm) Fe 344 1 Ni 223 0.13 Cr 205 0.19 Nb 463 270 O 221 <25

EXAMPLE 4

Ingots obtained from electron beam melting are cold worked and annealedto produce target blanks. Initial ingot breakdown is done via acombination of side and upset forging. After surface machining, theforged ingot is cut into pieces, which are further cold rolled intoblanks. Two rolling temperatures are considered: room temperature andliquid nitrogen temperature. The former is called cold rolling, whilethe latter is referred to as cryogenic rolling. The reduction at rollingis in the range of 70 to 90%. The rolled blanks are annealed in an inertatmosphere or vacuum under different conditions to obtain the desiredmicrostructure and texture.

EXAMPLE 5

Blanks with fine grains and desired texture are machined to obtain thefinal finish and bonded to copper or aluminum backing plates. Aschematic of the target produced is shown in FIGS. 4A and 4B.

In the foregoing discussions, it is apparent that various changes andmodifications may be made within the preview of the invention.Accordingly, the scope of the invention should be limited only by theappended claims.

1-30. (canceled)
 31. A high purity tantalum powder comprising greaterthan or equal to 99.95 weight percent (wt %) tantalum on a total metalsbasis, less than 500 parts per million by weight (ppmw) total metallicimpurities, less than 50 ppmw tungsten or molybdenum, and less than 10ppmw niobium.
 32. The powder of claim 31 comprising less than 20 ppmwtungsten or molybdenum.
 33. The powder of claim 31 comprising less than5 ppmw each of tungsten and molybdenum.
 34. The powder of claim 31comprising less than 5 ppmw total of tungsten and molybdenum.
 35. Thepowder of claim 31 comprising less than 20 ppmw total of tungsten,molybdenum, and niobium.
 36. The powder of claim 31 comprising less than5 ppmw total of tungsten, molybdenum, and niobium.
 37. The powder ofclaim 31 comprising less than 3 ppmw niobium.
 38. The powder of claim 31comprising less than 100 ppmw oxygen.
 39. The powder of claim 31comprising less than 25 ppmw oxygen.
 40. The powder of claim 31comprising less than 10 parts per billion by weight (ppbw) uranium andthorium.
 41. An ingot formed from the powder of claim
 31. 42. Asputtering target blank formed from the powder of claim
 31. 43. Asputtering target blank comprising greater than or equal to 99.95 weightpercent (wt %) tantalum on a total metals basis, less than 500 parts permillion by weight (ppmw) total metallic impurities, and less than 5 ppmwtotal of tungsten, molybdenum, and niobium.
 44. The blank of claim 43comprising less than 25 ppmw oxygen.
 45. The blank of claim 43comprising less than 10 parts per billion by weight (ppbw) uranium andthorium.